1. Field of the Invention
This invention relates to a control arrangement for a switched mode power supply and in particular relates to a control arrangement for interfacing a convertor in a switched mode power supply with the mains power supply to the switched mode power supply for ensuring that the waveform of the input current from the mains power supply is corrected to be substantially the same waveform as the input voltage from the mains power supply.
2. Description of the Prior Art
Switched mode power supplies, which are primarily used for supplying large currents at low and medium voltages, have recently been the subject of intensive development in an effort to produce high efficiency power supplies which are both small and light weight. In such systems, a mains input is fed, via a RFI filter, to a rectifying and capacitive smoothing circuit which supplies the input to a DC/DC convertor, the convertor providing a regulated DC output. The general operation of such a switched mode power supply is well known and will not be described in detail herein.
It is a problem, however, with such arrangements of switched mode power supplies that a current is drawn only for a relatively short duration at the peak of the input voltage waveform. The apparent input power factor of prior art arrangements, i.e. unit output power plus convertor losses to mains input power, can be less than 0.5. Consequently, in order to handle the increased RMS current required, it is necessary to over dimension the input cabling and, in the case of an uninterruptable power supply system, the mains input source itself.
Hitherto, this problem has been addressed by providing the following control arrangement which comprises a switching means in parallel with the capacitor of the smoothing circuit. A feedback loop is used to sense the output voltage to the convertor, compare it with a reference level and feed the resulting error signal to a multiplier. A signal derived from the voltage input from the rectifier is also provided to the multiplier so that the resulting signal from the multiplier is a series of full wave rectified semi-sinusoids at mains frequency, whose amplitude is determined by the error signal. This signal is then compared to a signal derived from the input current to the convertor and the resulting error signal is fed to a clocked modulator to control the duty ratio of the switching means. Consequently, the average input current may be constrained to have substantially the same waveform as the input voltage. Additionally, the amplitude of the input current may be determined by the gain of the comparator which compares the output voltage to the convertor with a reference voltage level. Such arrangements are, however, complex and somewhat costly because of the design philosophy which requires the provision of an analogue multiplying element to allow the feedback signal to be modified by the input voltage waveform.